The right thrust angle for the RV7A is 1º10'.  I converted this to
1.66 degrees.  Then drew it out and measured the off set at the
firewall (37.5 inches) to be .842 inches.  Is this correct?

What is the firewall to prop distance?

SIN ( 1.66 degrees ) = offset / firewall to prop distance.

offset = SIN ( 1.66 degrees ) X firewall to prop distance.

SIN ( 1.66 degrees ) = .0289 times 37 inches  = approx. 0.87815002 :)